Recent advances in the superconducting transition temperature of various oxide materials has provided the opportunity for applications in radio frequency, microwave and other electronic technologies. Considerable progress has been made in a number of fabrication technologies related to forming these oxide superconductors into various electronic devices.
The technique of laser evaporation for deposition of thin films has been applied to a large class of materials ranging from polymers to semiconductors and dielectrics. For most present electronic device applications such as radio frequency and microwave technology, thin films are proving to be the form of superconducting oxide most useful. As a result, much of the work relating to the use of high temperature superconductors in microelectronic applications has focused on the growth of high quality thin films. Such films can serve as ground planes or can be patterned into a microwave circuit. High quality superconducting films can have significantly lower values of surface resistance than copper or gold films. This low surface resistance is important for making high-performance thin film microwave circuits such as filters, resonators and delay lines.
While various methods are available for producing superconducting thin films, one of the better methods is pulsed laser deposition, described first by D. Dijkkamp at al., Appl. Phys. Lett., 51, 619-621 (1987). This method provides thin films of superconducting material having correct stoichiometry and high superconductivity transition temperature, T.sub.c. In addition to the high deposition rate provided by this method and relatively simple apparatus required, the film can be grown at low substrate temperatures, as discussed in Koren et al., Appl. Phys. Lett., 56, 2144-2146 (1990). A major drawback of this technique is that the deposited film contains a large number of particles, commonly referred to as particulates, typically on the order of 0.5 .mu.m to 2 .mu.m in width and are commonly present in a density of 110-140 particulates per 1000 .mu.m.sup.2. The presence of these particulates limits the use of the film in most applications where microscopic patterns must be printed thereon, e.g., microwave circuits. Koren et al. has speculated that these undesirable particulates may result from loosely connected target flakes, splashing of molten material, or deep target subsurface heating that leads to an explosion of bubbles and condensation in the high density plume that results from the laser ablation of the target.
Several techniques have been devised to reduce the size and/or number of particulates in these films. One described by Koren et al. involves the use of a second laser beam perpendicular to the first to reduce the size of the particulates. Another involves using a target that is moved at a velocity comparable to the fragments in order to eject the particulates, Research Disclosure, 32397, March 1991, page 209.
While these methods have met with some success, they involve more expensive and complex apparatus than conventional laser ablation and do not produce sufficiently particulate-free films for demanding applications. The present invention provides a method for producing films wherein the amount of particulates is reduced by orders of magnitude rendering such films of superior utility as circuit elements.